Measurement of absolute tracer concentrations in tissue sections by using digital imaging fluorescence microscopy. Application to the study of plasma protein uptake by the arterial wall.

Digital imaging fluorescence microscopy (DIFM) of tissue sections was used to quantify uptake of labelled plasma proteins by the arterial wall. Several aspects of the measuring system were investigated so that absolute tracer concentrations and their local variation could be derived from digitized images. These investigations may be relevant to other studies employing DIFM. Nonlinearities were found to arise from offsets in the video digitizers, from background fluorescence and stray light within the microscope and from the transfer characteristics of the intensified CCD camera. Camera gain controls showed complex behaviour. Camera output fell substantially for several hours after switching on and was affected by room temperature. Large spatial variations in response were caused by the geometry of the microscope optics and by the image intensifier. However, the ratios between areas were not affected by light intensity or camera gain settings. Measured intensities were independent of the depth-wise location of fluorophores within tissue sections but they were affected by the emission from objects outside the measuring area. Photobleaching of tracer varied significantly over the range of excitation intensities and durations used but was not concentration dependent. Methods used to correct these effects and obtain a uniform, linear and constant relationship between concentration and grey level are described. Using the system and appropriate corrections, in vivo uptake of sulphorhodamine-B-labelled serum albumin by the rabbit aortic wall was investigated. Results obtained for the mean uptake of tracer and its local variation were quantitatively similar to those previously obtained with nonmicroscopic methods.